Apparatus for applying powder coatings to articles



N. R. WALLIS Aug. 26, 1969.

APPARATUS FOR APPLYING POWDER coATINcs'To ARTICLES Filed May 15, 1967 Iuveurom NEIL FLLDDLIJH (J M-ls .ll Iiiiiiixllg A- Thug/5 United States Patent 3,463,126 APPARATUS FOR APPLYING POWDER CGATINGS T0 ARTICLES Neil Rudolph Wallis, Cariad, Goring-en-Thames, England, assignor to Aerocoat 3A., Geneva, Switzerland Filed May 15, 1967, Ser. No. 638,328 Claims priority, application Great Britain, May 26, 1966, 23,514/ 66 Int. Cl. B051: 5/02; BOSb 5/08 US. Cl. 118-626 11 Claims ABSTRACT 0F THE DISCLGSURE An electrostatic gun for powder material having a rotatable hollow dome-shaped member provided with a plurality of first slots around the periphery of the dome in which the powder leaves the member in a direction generally perpendicular to the axis of rotation and a plurality of second slots in the central portion of the dome in which the powder leaves the member in the general axis of rotation.

This invention relates to apparatus for applying powder coatings to articles under the influence of an electrostatic field.

It is an object of the invention to provide powder coating apparatus capable of applying powder to articles of varying shapes and sizes. It is a further object of the invention to provide powder coating apparatus having a spray pattern which is controllable by simple means.

The invention consists in apparatus for applying powder coating material to an article, including a hollow rotatable member, means for supplying said coating material to the interior of said hollow member, a variable speed motor for rotating said member and means for maintaining an electrostatic field between said member and said article, wherein the rotatable member is provided with a first aperture or a plurality of first apertures through which said material can leave the member in directions generally perpendicular to the axis of rotation of the member and with a second aperture, or a plurality of second apertures, through which said material can leave the member in the general direction defined by the said axis of rotation.

Preferably, the hollow rotatable member is dome-shaped and the exterior of the dome may be, for example, a surface of revolution formed by rotating a semicircle about an axis perpendicular to the axis of the semicircle. The hollow member may be moulded or fabricated in one piece and in this case a plurality of first apertures are provided and these may be, for example, in the form of a plurality of slots extending about the periphery of the dome in the region of the edge that is furthest from the axis of rotation. The second apertures are preferably arranged asymmetrically in the surface of the dome and are located in a central portion of the dome so that they face generally forwardly. Some of the second apertures may be in the form of slots and one or more of the other second apertures may be in the form of a circular hole or holes.

In apparatus in accordance with the invention, the powder coating material will leave the hollow member axially through the second apertures when the member is stationary or is rotated slowly, whereas it will leave the member radially if the motor is driven at a higher speed. Any desired combination of axial and radial travel can be achieved by the selection of the appropriate inter mediate speed of the motor.

To provide even better control of the characteristics of the spray pattern the rotatable member is preferably surrounded by an annular air stream produced by a plurality of elementary jets of air in accordance with my patent specification No. 1,024,892. At least some of the jets of air converge at small angles towards the axis of rotation of the hollow member and these angles are varied in a predetermined sequence around the circumference of the circle on which the jets are located. Variation of the air pressure in combination with variation of the speed of rotation of the hollow member, possibly also together with variation of the electrostatic potential enables powder to be applied to articles of widely varying shapes and sizes. The air stream is particularly useful in causing powder to enter depressions and cavities which are electrostatically screened.

The invention will now be described with reference to the accompanying diagrammatic drawings, in which:

FIGURE 1 is an overall side View of a manual electrostatic powder gun in accordance with the invention;

FIGURE 2 is a view of the front end of the gun illustrated in FIGURE 1 on a larger scale, partly in section;

FIGURE 3 is a sectional view of the head of the gun illustrated in FIGURES 1 and 2 on a still larger scale;

FIGURE 4 is a front view of the rotatable member of the gun illustrated in FIGURE 1; and

FIGURE 5 is a rear view of the rotatable member illustrated in FIGURE 4.

Referring now to FIGURE 1, it will be seen that the gun includes a barrel 21 in the form of an insulating tube. Surrounding the barrel 21 is a clamp 22 by means of which a handle 23 is secured to the barrel. The handle 23 includes two triggers 14 and 24 controlling respectively two microswitches 15 and 25. The microswitches are connected to a cable 26 which extends to the back of the gun.

The head of the gun comprises a shroud 27 and a rotatable member 28. Both the cup and the rotatable member consist of insulating material but it is desirable that conductive or semi-conductive material should be painted on or incorporated in the insulating material to give the shroud and the rotatable member a very high electrical resistance. Air is supplied under pressure to the shroud by means of an air line 29 which fits on to an air connector 30. Powder coating material is supplied to the interior of the rotatable member 28 by means of a pipe 31 which is bifurcated into two end sections 12 and 32.

As can be seen in FIGURE 2, the member 28 is rotated by means of an air motor 33 having a driving spindle 35. Air is supplied under pressure to the air motor 33 by means of an air line 34 which extends through the barrel of the gun. The air motor 33 also serves as the electrical connection between the high-voltage supply and the rotatable member 28. If the air motor 33 is constructed of insulating material, a conductive path is provided therethrough. The high-voltage supply is provided by means of a cable 36 which is preferably in the form of a flexible tube filled with an electrolyte 6 in accordance with my United Kingdom patent specification No. 1,030,819. As described in that specification, the end of the flexible tube is enclosed in a rigid tube 39 and is sealed by means of a, plug 8 incorporating an electrode 7. The plug 8 is connected to a spring 9 and thence to a conductive cap 10 which bears against the motor 33.

A nylon block 40 is secured in the end of the barrel 21 by means of a screw 41. This block includes a plurality of tapped holes by means of which the shroud 27 is screwed to the block. The block also includes a central hole through which the spindle 35 passes.

The head of the gun is shown in more detail in FIG- URE 3 and it will be seen that the shroud 27 consists primarily of two parts 46 and 47 which are secured to one another by means of plurality of nylon screws 48. The rear part 46 includes a threaded hole into which the airconnector 30 is screwed. The inner end of the air connector includes a pair of holes of which one is visible at 49 and which communicate with an axial drilling along the length of the connector. This arrangement enables air fed to the connector 30 to pass into an annular channel 50 cuit in the front face of the portion 46. The portion 46 is also provided with a hole 51 through which the spindle 35 passes.

The front portion 47 is generally in the form of a shallow truncated cone and includes a generally annular passage 52. This passage extends towards the front face of the truncated cone in which a plurality of very fine holes such as 53 and 54 are drilled. The holes 53 and 54 are located on the circumference of a circle whose centre lies on the axis of rotation of the member 28 and each hole extends through an appreciable thickness of the insulating material from which the front portion 47 is made so that it provides a passage the length of which is considearably greater than its diameter and which thus acts as a minute air jet. The axis of the air jet 53 is represented by the dashed line 55 and the axis of the air jet 54 is represented by the dashed line 56. The angles made by the axes of successive jets with the axis of rotation of the member 28 vary in a predetermined sequence around the circumference of the circle on which the holes are located. In one particular example, this sequence is 8, 11, 8, 5, 11, and so on and in this particular example there are 88 holes made with a drill No. 80 (0.135").

The channel 52 is provided with a shoulder 57 on which is located a ring 58 in which a number of holes 59 are drilled, the total area of these holes being slightly greater than the total area of the holes such as 53 and 54. The front portion 47 is also provided with a further shoulder 60 and on this shoulder is located a further ring 61. This ring is provided with a number of holes 62 and the total area of the holes 62 is made slightly greater than the total area of the holes 59 so that a back pressure is maintained in the cavity between the two rings as well as in the cavity formed between the ring 58 and the forward end of the channel 52. The ring 58 is held in position on the shoulder 57 by means of a plurality of springs such as those shown at 65 and 66.

The rotatable member 28 which consists of an insulating or semi-conducting material, is generally dome-shaped but includes a central boss 70 projecting outwardly from the interior 72 of the dome. The boss 70 is provided with a metal insert 63 by means of which the rotatable member is connected to the driving spindle 35 of the motor. In addition, a metal washer 64 is provided between the spindle 35 and the insert 63 and the washer projects slightly beyond the edge of the insulated boss 70.

The periphery of the rotatable member includes a plurality of curved slots 71 extending between interior and exterior of the domed surface. In addition, the forward face of the rotatable member is provided with three slots 73 and a circular hole 74. As can be seen from FIGURES 3 and 4, the slots 73 and the hole 74 are asymmetrically arranged and one of the slots 73 is extended substantially as far as the axis of rotation of the member 28 by removing material from the forward end of the boss 70.

In use, the article to be coated is earthed and the gun is connected to a high voltage generator and associated control equipment by means of a multiple cable which includes the airline 34 for the motor, the powder line 31, the high voltage cable 36, the trigger control cable 26, and the airline 29 for the air supply to the shroud. The trigger cable 26 connects the two microswitches 15 and 25 to control equipment which is mounted on a high voltage radio frequency generator connected to the main supply. The control equipment is connected to a bank of remote controlled valves which serve to couple a compressed air supply to the airlines 29 and 34, and also to a powder supply system. Two separate valves together with associated manual setting controls are provided in the airline 34 and the arrangement is such that one valve is opened when the trigger 14 is pressed and the other valve is opened when the trigger 24 is pressed. Thus, it is possible to select two different speeds for the motor 33 by pressing the appropriate trigger 14 or 24. The actual speed selected by means of each trigger may be set up by means of the manual control means associated with the respective remote control valves and one speed may, for example, be between 25 and 50 revolutions per minute while the other is between and 200 revolutions per minute.

Preferably, the arrangement of the control circuit is such that when either trigger on the gun is pressed, air is first supplied under pressure to the shroud and to the motor to cause the rotatable member 28 to revolve. When the member 28 is revolving at a sufiicient speed the valve to the powder supply is opened so that powder is delivered to the bifurcated pipe 31. Pressure on either trigger also operates a relay in the high voltage generator so that a high potential is applied to the cable 36. This potential reaches the washer 64 through the connector 10 and the motor 33. Accordingly, powder particles are electrostatically charged when they leave the hollow member 28 and their path of travel is influenced by the electrostatic field between the member and the article to be coated. The majority of the powder travels towards the article under the influence of the electrostatic field but the dimensions of the spray pattern produced on the article depend on the speed of rotation of the hollow member. When the member is rotating slowly, substantially all the powder leaves the hollow member axially through the slots 73 and the hole 74. In these circumstances, the spray pattern is in the form of a small circle. If on the other hand, the motor is set to rotate at maximum speed, substantially all the powder will leave the member through the slots 71 and will have a substantial radial velocity. The electrostatic field will attract these particle towards the article to be coated but the spray pattern may be, for example, in the form of an annulus of relatively large outer diameter. At intermediate speeds a succession of different patterns may be produced between these two extremes.

In addition to the centrifugal and electrostatic effects referred to above, the powder particles are also controlled by the annular air stream formed by the jets of air tom the shroud. Variation of the air pressure in combination with variation of the speed of rotation of the member 28 and possibly also together with variation of the electrostatic potential enables the powder to be applied to articles of widely varying shapes and sizes. The air stream from the shrould is particularly useful in causing powder to enter depressions and cavities which are electrostatically screened. Further, the air stream from the shroud is elfective to fill in the central void in the annular spray pattern which would otherwise be produced when the member 28 is rotating at high speed.

What I claim as my invention and desire to secure by Letters Patent of the United States is:

1. Apparatus for applying powder coating material to an article, including a hollow dome-shaped rotatable member, means for supplying said coating material to the interior of said hollow member, a variable speed motor for rotating said member, and means for maintaining an electrostatic field between said member and said article, wherein the rotatable member is provided with a plurality of first slots through which said material can leave the member in directions generally perpendicular to the axis of rotation of the member, said first slots extending around the periphery of the dome in the region of the edge that is furthest from the axis of rotation, and a plurality of second slots, through which said material can leave the member in the general direction defined by the said axis of rotation, said second slots being located in the central portion of the dome so that they face in directions having a component parallel to the axis of rotation of the dome.

2. Apparatus as claimed in claim 1, wherein the domeshaped member includes at least one generally circular hole having an axis parallel to the axis of rotation of the dome.

3. Apparatus as claimed in claim 1, wherein at least one of the second slots extends substantially as far as the axis of rotation of the dome.

4. Apparatus as claimed in claim 1, wherein the rotatable member includes a central boss projecting rear- Wardly from the interior of the dome.

5. Apparatus as claimed in claim 4, wherein the rotatable member consists of an insulating or semi-conducting material but is provided with a metal insert in the boss by means of which the rotatable member is connected to the driving spindle of a pneumatic motor.

6. Apparatus as claimed in claim 5, wherein a metal washer is provided between the motor spindle and the metal insert and projects slightly beyond the edge of the insulated boss.

7. Apparatus as claimed in claim 1, wherein the rotatable member is at least partially surrounded by a shroud provided with a plurality of holes through which jets of compressed air issue to form a stream of air directed towards the article to be coated.

8. Apparatus as claimed in claim 7 wherein the jets of air converge towards the axis of rotation of the rotatable member.

9. Apparatus as claimed in claim 8, wherein the angles made by the axes of the jets vary in a predetermined sequence around the circumference of the circle on which the holes are located.

10. Apparatus as claimed in claim 1, wherein the rotatable member is located at the forward end of a tube of insulating material provided with a handle.

11. Apparatus as claimed in claim 10, wherein means are provided on or in association with said handle for controlling the speed of rotation of said variable speed motor.

References Cited UNITED STATES PATENTS 3,063,642 1 1/ 1962 Point 118--627 XR 3,117,029 l/ 1964 Hines 23915 3,085,749 4/ 1963 Schweitzer et al. 239-15 XR 3,275,239 9/ 1966 Oesterle 23915 3,346,412 10/ 1967 Siegenthaler 117--96 FOREIGN PATENTS 275,857 6/ 1965 Australia. 1,178,658 12/1958 France.

PETER FELDMAN, Primary Examiner US. Cl. X.R. 

